Control system including a control circuit and sensor, and method for operation of the control system

ABSTRACT

A controller includes a control circuit. The control circuit includes a forward path that includes an input and an output, a feedback path coupled to the output and to the input, and a sensor that is between the input and the output. The sensor generates a sensor signal based on an input signal applied to the input. The forward path generates an output signal based on the sensor signal. The output signal is sent along the feedback path to the input of the forward path. The controller also includes a detector that obtains an intermediate signal from the forward path between the input and the output. The detector generates a control signal using the intermediate signal. The forward path includes a control device that limits the output signal to a predetermined value. The detector controls the control device using the control signal.

TECHNICAL FIELD

The invention relates to a controller with a control circuit thatcontains a feedback path coupled back to the feedback input of thecontrol circuit, and with a sensor that is arranged in the controlcircuit and emits a sensor signal at its output, which is converted intoa feedback signal and routed back to the feedback input of the controlcircuit.

The invention further relates to a method for operating the controller.

BACKGROUND

Such control circuits are known. An input signal of the control circuitis usually routed to a summing point, whose output leads into thecontrol path that emits the output signal of the control circuit. Theoutput signal is measured and coupled back with a negative sign to thefeedback input of the summing point, i.e., the input of the controlcircuit. This makes it possible to control the output signal of thecontrol circuit as a function of its input signal in compliance with therequirements and control characteristics pertaining to the controlcircuit.

The output signal of the control circuit is measured by means of asensor selected based on the variable to be physically acquired. Thissensor can be arranged either in the control path itself or in thefeedback path. The control circuit is closed via the feedback signal,which forms in the feedback path, and is superimposed with a negativesign on the input signal.

Sensors for physical variables, e.g., magnetic or electric fields, orfor mechanical or chemical variables, exhibit a sensor-typicalcharacteristic for the output signal as a function of the input variableto be measured. These characteristics are often linear only in a smallregion, the operating range of the sensor, and exhibit a nonlinearcharacteristic outside this region between the input variable to bemeasured and the output signal of the sensor. This results innonlinearities that must be specially considered in a control circuit,and are often difficult to correct.

In certain instances, the characteristic of a sensor proceeds in anonlinear manner, wherein the sensitivity, i.e., the output signal ofthe sensor, no longer increases as the measuring variable rises, but thesensitivity tapers off again after a maximum has been exceeded as themeasuring variable continues to rise. Sensitivity follows the oppositepattern by increasing again after falling below a minimum as themeasuring variable drops. This behavior is observed, for example, in theHNC 1001/1002 magnetoresistive sensor made by Honeywell. At a magneticfield of 0 Oe, this sensor exhibits virtually no output voltage. In arange of up to 5 Oe, there is a linear dependence between the magneticfield and output voltage. Sensitivity tapers off as the magnetic fieldcontinues to rise, i.e., the output voltage of the sensor no longerrises to the same extent. At roughly 11 Oe, the sensitivity curve peaks.At higher magnetic flux densities, the output voltage of the sensortapers off toward 0. Negative flux densities of the magnetic fieldproduce a mirror image curve.

In a general controller, in particular in devices with magnetoresistivesensors, the nonlinearities caused by the sensitivity curve of thesensor negatively impact the properties of the closed control circuit.In overload situations involving a sharp rise in the input variable, thesignal output of the sensor might not emit a higher signal, but insteada lower signal not corresponding to the input signal to be measured.This means that the negative feedback loop might not be able to correctthe control circuit any longer in these extreme operating situations.Such an effect is referred to as foldback, and is undesired in allapplications.

SUMMARY

The object of the invention is to indicate a device and method of thekind mentioned at the outset in which nonlinear changes in thesensitivity of a sensor are detected, and reliable operatingcharacteristics are enabled.

This object is achieved according to the invention by the features inclaim 1 and claim 8.

The subclaims describe embodiments of the invention.

The invention provides that a change, in particular a deterioration, inthe sensitivity of the sensor be detected and/or measured. As soon assuch a change in sensitivity is detected or a prescribed level isexceeded, the output of the sensor is set to a predetermined value,preferably corresponding to the maximum measuring value. Thispredetermined value is preferably clamped, i.e., retained.

The foldback effect can be avoided in this way. In addition, irregularoperating modes of the control circuit can be displayed or madeapparent.

As soon as the sensor returns to an operating mode lying within theallowable sensitivity range, the clamp imposed on the predeterminedvalue can be advantageously lifted, and the sensor can perform regularlyonce again.

The controller according to the invention provides an error signalgenerator that generates an error signal and feeds it into the controlcircuit, wherein the error signal is superposed on the useful signal ofthe control circuit. This useful signal can be the applied signal of thecontrol path before the feeder node for the error signal. Also providedis a detector, which monitors the control circuit and establishes agauge for the change in sensitivity or deterioration in sensitivity ofthe sensor. The output side of the detector feeds a control device ofthe control circuit, which sets or clamps the output signal of thecontrol circuit to the predetermined value as a function of the outputsignal of the detector.

A device according to the invention makes it possible to utilize theamplification of the closed control circuit taking into account thesensor arranged in the control circuit, without having to open thecontrol circuit, e.g., to measure the sensor sensitivity.

In a regular operating mode, the control circuit corrects the superposederror signal. The magnitude of the error signal and the location of thefeeder or summing point at which the error signal is fed into thecontrol circuit is here selected based on the design rules of thecontrol circuit in such a way that the closed feedback circuit minimizesthe error signal at the output of the controller to a negligible signalvalue relative to the useful output signal of the control circuit.

Given a deterioration in sensor sensitivity, the circuit amplificationof the control circuit changes, and the error cannot be corrected anylonger in the usual manner. This change can be detected by comparing thesignals of the control circuit at the feeder node for the error signalbefore and after feeding in the error signal.

If the control circuit functions regularly in a linear region, theoutput of the feeder node for the error signal will not correspond tothe input of this feeder node, and given a deteriorated loopamplification, the output of the feeder node for the error signal willcorrespond to the input of the feeder node. The comparison would becorrespondingly opposite were the signal comparison to take place at theinput of the feeder node for the error signal, and not its output.

In order to be able to compare the respective signals before and afterthe superposed error signal, the corresponding signal is measured andstored before feeding in and measuring the error signal. To this end,the detector preferably has a memory and comparator, which compares asignal of the control path with a signal stored in the memory.

The comparator is preferably connected with a decision circuit, whichgenerates the output signal of the detector connected with the controldevice.

In an advantageous embodiment, the control device has a clamp circuit,which clamps the output signal of the control path to the predeterminedvalue.

In another advantageous embodiment, the detector contains a signal levelcomparator and/or a signal sign comparator, whose input is connectedwith the control circuit, and whose output is connected with thedecision circuit.

The output of the decision circuit is connected with the control deviceon the one hand, and with the error signal generator on the other, whoseother input is connected with a time signal generator. Depending on thesignals of the decision circuit, this is why the control device isclamped to the predetermined value on the one hand, and the error signalgenerator is activated or deactivated on the other.

The signal sign comparator is provided in particular in bipolar sensorsystems, i.e., in sensor systems that measure positive or negativesigned measuring variables. A sign is found in this manner, so that theerror signal can be fed into the control circuit with the correspondingpolarity. In general, it must here be stated that the error signal,typically a rectangular signal, is injected with a polarity opposite thepolarity of the output signal of the control circuit.

The signal level comparator is advantageous in particular in cases wherethe sensor system has sufficiently high loop amplification with thecontrol circuit exposed to overload conditions to clamp the outputsignal of the control circuit to the predetermined value. In this case,the output of the detector can be deactivated as an option. The signallevel comparator monitors the signal amplitude of the control circuit todetermine whether the loop amplification is sufficiently high based onthe design criteria of the control circuit.

In a particularly advantageous embodiment of the invention, the sensoris a magnetoresistive sensor suitable for acquiring a magnetic field.This type of sensor makes it possible to configure the control circuitas a current measuring system.

The invention will be described in greater detail below based onexemplary embodiments shown on the figures in the drawing. Shown on:

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a wiring diagram of the controller according to the invention;

FIG. 2 a is a controller with a magnetoresistive sensor for currentmeasurement, and

FIG. 2 b is a wiring diagram of the controller according to FIG. 2 a.

DETAILED DESCRIPTION

FIG. 2 a shows a controller with a current measuring sensor consistingof three main components. The first component is a current transformercontaining a core K with an air gap L. Accommodated on the core is aprimary winding W1, which carries a current I1. In addition, the core Kaccommodates a second winding W2, through which the current I2 flows onthe secondary side. The second component of the controller is themagnetic sensor S, which in this case is designed as a magnetoresistivesensor, and is situated in the air gap L of the core. The thirdcomponent of the controller is a negative feedback path, which connectsthe sensor S with the secondary winding W2 of the transformer by way ofan amplifier G. The output side of the secondary winding W2 is connectedby a resistor R2 with a reference potential VB, e.g., ground.

The input current I1 flowing in the primary winding W1 of the currenttransformer generates a magnetic flux in the air gap L. This magneticflux is acquired by the sensor S, and calibrated to zero by negativelycoupling back the control circuit. The control circuit here sets thecurrent I2 passing through the secondary winding W2 in such a way thatthe magnetic flux generated by the current I2 is directionally oppositeand quantitatively identical to the magnetic flux generated by thecurrent I1 in the air gap. The current I2 passing through the secondarywinding is the gauge for the magnitude of current I1 in the primarywinding.

The sensor S, a magnetoresistive sensor in the exemplary embodiment,exhibits the parasitic property of decreasing sensitivity that becomeszero if the magnetic flux density in the air gap L exceeds asensor-specific maximum field. The maximum value of the magnetic fieldstrength is a property of the sensor that varies specific to the sensor.The described parasitic property of the sensor detracts from theproperties of the closed control circuit of the control sensorcontroller.

Under overload operating conditions, in which a high current I1 flows inthe primary winding, the current I2 in the secondary winding cannot befurther increased, since the output of the current sensor S has alreadyreached its maximum value. It is even possible that the initial value ofthe current sensor will drop again given a rising magnetic fieldstrength. This means that the magnetic flux in the air gap L can nolonger be compensated through negative back coupling with the help ofthe secondary current I2, so that a value for the magnetic flux in theair gap L is not equal to zero. For this reason, the magnetic flux inthe air gap of the current transformer will rapidly increase as theprimary current I1 continues to rise, since the current I2 can no longercompensate. As a result, the magnetic flux in the air gap will rise to avalue exceeding the maximum value of the sensor allowable formeasurement. The sensor either has lost or will lose its sensitivity. Ina magnetoresistive sensor of the kind described in the data sheetpublication of Honeywell alluded to at the outset, magnetic fluxes canbe accompanied by a foldback effect, in which the output signal of thesensor becomes even smaller than at lower magnetic field strengths. Sucha foldback effect puts the controller in an uncontrollable state, whichis undesired for any application.

FIG. 2 b shows an equivalent circuit diagram of the controller accordingto FIG. 2 a. As a variable to be measured, the current I1 generates themagnetic flux B1, which is passed to a summing unit Sr as the inputsignal of the control circuit R. A magnetic flux is acquired by thesensor S at the output side as an output signal of the summing unit SR,and converted into the secondary current I2 of the control circuit bymeans of an amplifier with the two elements G1 and G2. In turn, thetransformer helps the secondary current I2 to generate the magnetic fluxB2, which is relayed back with a negative sign to the second input ofthe summing unit SR. The element G shown as an amplifier on FIG. 2 a isrealized by two elements G1 and G2 on FIG. 2 b in order to make theinvention easier to understand based on FIG. 1.

FIG. 1 shows a controller according to the invention based on theexemplary embodiment of the current measuring controller, a basicdescription of which has already been given based on FIG. 2. The sameelements as on FIG. 2 b are marked with the same reference numbers onFIG. 1. As opposed to FIG. 2 b, a summing unit SF connected at one inputwith the output of the amplifier G2 is arranged between the twoamplifier elements G1 and G2 on FIG. 1. The other input is connectedwith an error signal generator F, which generates an error signal andfeeds it into the summing unit SF. The error signal of the error signalgenerator F is generated when the output of the error signal generator Fis released by a decision circuit E and/or a time signal generator TC.The error signal can be rectangular. The output of the summing unit SFis routed to the amplifier G1 on the one hand, and to a detector D onthe other. The output side of the detector D is coupled back to theerror signal generator F on the one hand, and to a control device KS onthe other, which is designed as a clamp circuit, and connected in theoutput circuit of the amplifier G1. The output of KS determines thecurrent I2 of the secondary winding of the transformer.

Under normal operating conditions with the control circuit not exposedto overload, the control device KS is bridged over, so that the outputcurrent I2 is formed by the output current of the amplifier G1.

During overload operation, the control device KS is actuated by thedetector D in such a way that the output current I2 is set and clampedto a predetermined value typically corresponding to the maximum outputcurrent value of the controller. This value exceeds the maximum outputcurrent allowed under normal operating conditions. In conjunction with arecorder or display, it is therefore possible to ascertain a regularoperating mode of the current measuring device, because the outputcurrent I2 is clamped to a current higher than allowed during measuringoperation.

The operating mode of the controller, i.e., the sensitivity of themagnetoresistive sensor S, is measured with the device according to FIG.1 by routing an error signal of the error signal generator F to thesumming unit SF, and having the error signal overlap the measuringsignal. The error signal is considerably smaller by comparison to themeasuring signal of the control circuit.

The injection point of the error signal can also be provided at anotherlocation in the control circuit. It is important that the error signalbe injected into the closed control circuit, so that is also correctedin the controller. This makes it possible to utilize the full loopamplification of the control circuit, and incorporate the error signalin the characteristic behavior of the sensor S in order to correct thesuperposed error signal using the secondary current I2 in the magneticfield.

The magnitude of the error signal generated by the error signalgenerator F is selected in such a way that the closed control circuitminimizes the error signal to an undetectable signal level at the outputof the current sensor.

Given a reduction in sensitivity of the sensor S, the loop amplificationof the control circuit decreases, and the error signal cannot be fullycompensated any longer. As opposed to the regular state in which theerror signal can be fully compensated, this state involving a reducedsensor sensitivity can be detected by comparing the respective signalsat the summing point for the error signal SF before and after injectingthe error signal. If the control circuit is operating under normaloperating conditions, the output of the summing unit SF will not followthe error signal, because the control circuit compensates for the errorsignal. However, the error signal does become measurable at the outputof the summing unit SF given reduced loop amplification. The situationis correspondingly opposite when the comparison is performed at theinput of the summing unit SF or at the output of the amplifier G2.

In order to now be able to compare the respective signal at the outputof the summing unit SF before and after superposing the error signal, atleast one of the signals must be intermediately stored, thereby enablinga comparison with the respective other operating state. The outputsignal of SF is intermediately stored in the detector D by storageelement SP, which occupies an input of the error comparator EC on theoutput side. The other input of the error comparator EC is connectedwith the output of the summing unit SF. The output side of the errorcomparator leads to a decision circuit E, which generates an outputsignal at a corresponding input signal in order to actuate the controldevice KS for clamping the output current I2.

In a controller incorporating a sensor system that allows bothpolarities, e.g., for oppositely oriented magnetic field strengths, thedetector D contains a sign storage element SC, whose input is connectedwith the output of the summing unit SF, and whose output is connectedwith the decider logic E. The sign storage element SC stores the sign orpolarity of the applied signal, so that the output of the decider logiccoupled back to the error signal generator F can be set in an overloadregion in such a way that the polarity of the error signal injected atthe node SF is set opposite to the sign of the output signal of thecontrol circuit.

Preferably provided as an additional element for the detector D is asignal level comparator LC, whose input is also connected with thesumming unit SF, and whose output is connected with the decider logic E.Under overload conditions in which the controller of the sensor systemstill has a sufficiently high loop amplification to set the output ofthe control circuit to its maximum value, the signal level generator LCensures that part of the detector D can be optionally deactivated. Inthis case, the decider logic does not have to generate a signal thatactivates the clamp circuit KS.

As depicted based on FIG. 1, it is especially advantageous tointermittently inject the error signal of the error signal generator Finto the node SF. This can be accomplished with a frequency in the kHzregion, so that the measuring process of the detector D lies in a rangeof a few microseconds. The advantage to the invention is that thisregion can always be clearly discerned in an overload region, and thatthe closed control circuit need not be opened for this purpose.

If the current I1 passing through the primary winding of the transformerdrops back down to a value that allows regular sensor operation orregular controller operation upon termination of the overload region,this fact is ascertained using the detector D, and the control device KSis again bridged over, or the clamp circuit deactivated.

It must be emphasized that the control arrangement according to theinvention involves a universal device or universal method. The inventioncan be used in any controller in which a sensor with parasiticproperties is incorporated into the control circuit. In a closed controlcircuit, the loop amplification of the system is used to check thesensitivity or accuracy of the sensor output. All parasitic errors ofthe control circuit, such as amplification changes, offset, noise orsuperposed error signals, are attenuated by the loop amplification withnegative feedback. Given a deterioration in properties of an element inthe closed control circuit, the superposed error signal is no longercompensated, and can therefore be used to detect the deterioration inproperties of the controller, and set the output of the controlleraccordingly.

1. A controller comprising: a control circuit comprising a closed loopcircuit, the closed loop circuit comprising: an input; an output; aforward path coupled to the input and to the output; a feedback pathcoupled to the input and to the output; and a sensor having asensitivity, the sensor being in the forward path or in the feedbackpath, the sensor for generating a sensor signal; an error signalgenerator that is external to the closed loop circuit, the error signalgenerator to generate an error signal and to provide the error signal tothe closed loop circuit such that the error signal is incorporated intoa useful signal of the closed loop circuit, wherein the error signal hasa preselected magnitude, the closed loop circuit being configured togenerate an output signal at the output, the output signal being basedon the sensor signal and the error signal, the output signal being sentalong the feedback path to the input; and a detector configured todetect a change in the sensitivity of the sensor, the detector beingcoupled to the forward path, the detector to generate a control signal;wherein the forward path comprises a control device, which is coupled tothe output, to limit an output signal at the output to a predeterminedvalue, the detector to control the control device using the controlsignal.
 2. The controller of claim 1, wherein the detector comprises: astorage device to store a measurement signal; and a comparator tocompare an intermediate signal to the measurement signal and to output acomparator signal, the intermediate signal being stored in the storagedevice.
 3. The controller of claim 2, wherein the detector furthercomprises: decision logic to receive the comparator signal and tocontrol the control device in accordance with the comparator signal. 4.The controller of claim 1, wherein the control device comprises a clampcircuit.
 5. The controller of claim 2, wherein the comparator comprisesat least one of a signal level comparator and a signal sign comparator.6. The controller of claim 1, further comprising: a time signalgenerator to generate a time signal output, wherein the error signalgenerator is configured to generate the error signal based on the timesignal output.
 7. The controller of claim 1, wherein the sensorcomprises a magnetoresistive sensor.
 8. A method of operating acontroller comprised of: a control circuit comprising a closed loopcircuit, the closed loop circuit comprising: an input; an output; aforward path coupled to the input and to the output; a feedback pathcoupled to the input and to the output; and a sensor having asensitivity, the sensor being in the forward path or in the feedbackpath, the sensor generating a sensor signal, the forward path generatingan output signal based on the sensor signal, the output signal beingapplied to the input coupled to the forward path via the feedback path;wherein the method comprises: generating an error signal that has apreselected magnitude, the error signal being generated outside theclosed loop circuit; incorporating the error signal into a useful signalof the closed loop circuit; obtaining a measurement signal from theclosed loop circuit, the measurement signal being obtained using adetector that is coupled to the closed loop circuit; generating acontrol signal that is indicative of a change in sensitivity of thesensor, the control signal being based on comparison of the measurementsignal and a stored signal; and applying the control signal to a controldevice in the closed loop circuit, the control device being coupled tothe output, and the control device limiting an output signal at theoutput to a predetermined value in response to the control signal. 9.The method of claim 8, wherein the measurement signal is stored in astorage device, and the comparison is performed using a comparator. 10.The method of claim 9, wherein the control signal is generated viadecision logic, the decision logic being controlled by an output signalfrom the comparator, the decision logic generating the control signal ifa predetermined criterion is satisfied.
 11. The method of claim 9,wherein the comparator comprises at least one of a signal signcomparator and a signal level comparator.
 12. The method of claim 10,wherein the error signal is generated based on an output of a timesignal generator and an output of the decision logic; and wherein themeasurement signal is based on both the sensor signal and the errorsignal.
 13. The method of claim 1, wherein the control signal comprise asignal output of the detector.
 14. The controller of claim 1, whereinthe sensor generates the sensor signal based on one or more inputsignals applied to the input of the forward path.
 15. The method ofclaim 8, wherein the sensor generates the sensor signal based on one ormore input signals applied to the input of the forward path.
 16. Acontroller comprising: a closed loop circuit comprising: an input; anoutput; a forward path coupled to the input and to the output; afeedback path coupled to the input and to the output; and a sensorhaving a sensitivity, the sensor being in the forward path or in thefeedback path, the sensor for generating a sensor signal, the sensorsignal being converted into a feedback signal and being applied to theinput via the feedback path; an error signal generator to generate anerror signal and to provide the error signal to the closed loop circuitsuch that the error signal is incorporated into a useful signal of theclosed loop circuit, wherein the error signal has a preselectedmagnitude and wherein the error signal generator is external to theclosed loop circuit; a detector, which is coupled to the closed loopcircuit, the detector being configured to detect a change in sensitivityof the sensor, the detector to generate a control signal based on thechange in sensitivity of the sensor; wherein the closed loop circuitfurther comprises a control device, the control device being coupled tothe output to limit an output signal at the output to a predeterminedvalue, the control device being controlled by the control signal. 17.The controller of claim 16, wherein the detector comprises: a storagedevice to store a measurement signal; and a comparator to compare astored signal to the measurement signal and to output a comparatorsignal.
 18. The controller of claim 17, wherein the detector furthercomprises: decision logic to receive the comparator signal and tocontrol the control device in accordance with the comparator signal. 19.The controller of claim 16, wherein the control device comprises a clampcircuit.
 20. The controller of claim 17, wherein the comparatorcomprises at least one of a signal level comparator and a signal signcomparator.